The present invention relates generally to a new and useful fuse structure having unique functional characteristics and capabilities.
The use of telecommunications circuits and networks containing those circuits is increasing significantly nationwide. Facsimile machines and computers, as well as other telecommunications devices, have become quite popular, thereby increasing consumer use of telecommunications circuits. In order to protect those consumers and their equipment from some of the dangers inherent in such circuits, regulatory agencies and testing laboratories, for example, have promulgated certain specifications for telecommunications circuits and elements thereof. One such element is a fuse.
Underwriters Laboratories has formulated a number of specifications for fuses used in telecommunications, as well as other circuits. One such specification states that a fuse must "fail gracefully" when it is subjected to a variety of current levels (i.e. amps) at 600 volts of alternating current (hereinafter "AC"). To fail gracefully, the fuse must open without fire, explosion, projectile production, or the like. The precise current level depends upon the type of fuse (e.g. 10,000 amps for a primary line fuse, 60 to 350 amps for a primary instrument fuse), but the industry has generally agreed that the appropriate current rating is 0.350 amps for all relevant telecommunications fuses. Additionally, a 22 gauge wire is in series with the fuse under the above-described conditions, and that wire must not fail under those conditions. Also, relating to sneak fuses, a sneak fuse must fail gracefully at 40, 7, and 2.2 amps at 600 volts AC. Additionally, the sneak fuse must protect a 1.6 amp slow-blow fuse connected in series with the sneak fuse. Thus, the 1.6 amp slow-blow fuse must be protected, under the above-specified conditions, from failure by the sneak fuse.
Another specification states that the fuse must open within 210 seconds of application of a current which is 150 percent of its rated current, which is 0.350 amps. Thus, to meet this second specification, the fuse must fail within a maximum of 210 seconds after application of a 0.525 amp current thereto. Yet another UL specification demands that the fuse carry its rated current for four hours without opening. Additionally, AT & T has a further specification which mandates that the fuse fail gracefully when subjected to a 6 amp current at 600 volts direct current (hereinafter "DC").
Construction of a single fuse to meet only one of these specifications, or even any two of the above-listed specifications, is relatively easy. However, none of the prior art fuses are singly able to meet any three of the specifications, let alone all four.
For instance, a fuse consisting of a wire filament embedded in a solid inorganic cement can carry a current of 0.350 amps for four hours, and can fail gracefully under the conditions of the first-mentioned UL specification. However, the fuse will not open within 210 seconds after application of a 0.525 amp current thereto because the fusible element therein will not reach an appropriate temperature for fusion. Worse yet, the entire fuse can become quite hot, often hot enough to melt a plastic casing in which the fuse may be mounted. This is unacceptable not only because the safety of telecommunications equipment, such as computers and the like, can be compromised, but, more importantly, because the safety of the human consumer may be compromised.
The new fuse structure, constructed according to the teachings of the present invention, is intended to solve some, if not all, of the problems presented by the corresponding fuses of the prior art. The fuse of the invention can also be employed in electric or electronic circuits other than those used in telecommunications.